Elevational distribution of forests and its spatiotemporal dynamics in subtropical China from 2000 to 2019

The subtropical region of China possesses abundant forest resources and features a mountainous terrain. Under the implementation of policies such as natural forest protection, the Grain for Green Project, and other initiatives since the beginning of the 21st century, coupled with climate change, the forest resources in this region have undergone significant changes compared to historical periods. In addition, forest resources distributing mainly in mountainous areas also implies that these changes may vary significantly with elevation. To explore the spatiotemporal dynamics of forest distribution in subtropical China since 2000, especially the trend of changes with elevation, we analyzed the data from two land cover products focusing on forest cover and forest types. We used a hierarchical approach, in which coarse-classification forest cover data with relatively small uncertainties impose constraints on forest type data with larger uncertainties, to achieve a reasonable balance between obtaining more details and reducing data uncertainty. We first divided the forest cover data into ‘unchanged’ and ‘changed’ categories. With the constraints by the forest cover results, we further analyzed the ‘unchanged’ and ‘changed’ forest types. The results indicated that, since the implementation of ecological engineering and management policies, 54% of the area in the subtropical region had maintained unchanged forest cover attributes over the past 20 years, which implied the good state of ecological environment. The results also showed that dynamic conversions existed in the long term between forests and lands for essential production needs like croplands. The elevational variations of forest cover suggested that the dominant changes came from the conversion between forests and croplands in low-elevation regions below 700 m, the conversion between forests and shrublands in mid-elevation regions of 700–1500 m, and the conversion between forests and grasslands in high-elevation regions above 2000 m. In the regions with unchanged forest cover, 96% exhibited unchanged forest types as well. Evergreen broad-leaved forests (EBF) were most widely distributed below 1700 m, while evergreen needle-leaved forests (ENF) dominated above 1700 m. There was still a large area of ENF and EBF undergoing dynamic conversions from/to transitional forest types such as mosaic of tree, shrub, and herbaceous cover (T-S-H) and mosaic of natural vegetation and cropland (NV-CRO). ENF almost unidirectionally transformed into T-S-H in low-elevation regions below 1000 m, and transformed from NV-CRO in mid- and high-elevation regions above 1000 m. EBF experienced an areal decrease and transformed into T-S-H in low-elevation regions, but the areal increase in mid- to low-elevation regions mainly transformed from NV-CRO. These variations with elevation may involve the impacts of specific human activities and climate change, and will provide a vertical dimension of information and perspectives for an in-depth exploration of the evolving ecosystem services of forest resources in subtropical China.